Recently, particular attention has been paid to ionic liquids as the base oil for grease used under high vacuum or ultra high vacuum conditions and under high temperatures (JP 2005-154755 A).
As compared with the lubricant oil, the grease is more suitable for lubricating the rolling bearings because of the advantages of easier attachment to metal materials, a smaller amount to be needed, less leakage and the like. The grease is a semi-solid lubricant comprising a base oil and a thickener. The thickener works to maintain the base oil and keep the semi-solid state.
There are many kinds of ionic liquids. When the polarity of the ionic liquid is strong, there is the drawback that the ionic liquid does not become thickened by the addition of a typical thickener (for example, lithium soap) and does not assume a semi-solid state.
The base oil of grease used in the outer space or under the conditions of high temperatures is required to be insoluble in water, and exhibit appropriate kinetic viscosities from the low temperature region to the high temperature region.